People who are HIV-positive can lower their risk for transmitting the virus to their partners by 90 percent by taking antiretroviral drugs, new research has found.

Antiretroviral drug therapy helps reduce HIV levels in the infected person’s blood, which in turn makes the person less infectious to others. The drugs are commonly taken in the United States by people with HIV, the virus that causes AIDS.

In the new research, published online May 26 in The Lancet, investigators studied more than 3,400 heterosexual couples in seven African countries. In each couple, one person was infected with HIV and the other was not. No one in the study was taking antiretroviral drugs.

Health workers counseled the couples about preventing HIV transmission and followed up with them for two years. During that time, periodic blood tests measured HIV levels in the blood, and people were referred to antiretroviral therapy when they became eligible.

Of those infected with HIV, 349 began the drug treatment. Only one of them, a woman, infected her partner after starting antiretroviral therapy, the study found.

That single transmission of the virus, however, indicates that couples in which one person is HIV-positive should continue safe-sex practices even if the infected partner is taking antiretroviral drugs, explained the study’s lead author, Deborah Donnell, a biostatistician with the Vaccine and Infectious Disease Institute at the Fred Hutchinson Cancer Research Center in Seattle.

“Our unique findings provide compelling new data for the HIV prevention field,” Donnell said in a news release from the center.